Zirconium (Zr) and hafnium (Hf) are often contained in phosphors, for example, used in the form of CaZrO3 as host crystals for adding light-emitting elements (see JP-A 8-283713), added to aluminate-based host crystals along with Eu for prolonging the afterglow of emission (see JP-A 8-73845), and added to rare earth oxychlorides or oxybromides along with Ce for improving the conversion efficiency of radiation-excited phosphors (see JP-A 11-349939).
However, few studies have been made on the luminescence and other properties of systems in which only zirconium or hafnium is added as a dopant to clear crystals which do not exhibit active optical characteristics in themselves.